How to determine the last element in a loop

I’ve managed to cobble together some code that parses a folder and displays all images located in that folder. My problem is that I need to attach a css class (class=”last”) to the very last image in the loop.

Any help much appreciated (by lots of folks on my mailing list)…

Here’s the code I have so far.

function get_images() {

	// only find files with these extensions
	$exts = 'jpg jpeg png gif';

	$str = ''; $i = -1; // Initialize some variables
	$folder = './wp-content/uploads';

	$handle = opendir($folder);
	$exts = explode(' ', $exts);
	while (false !== ($file = readdir($handle))) {
		foreach($exts as $ext) { // for each extension check the extension
			if (preg_match('/\.'.$ext.'$/i', $file, $test)) { // faster than ereg, case insensitive
				//$str .= $file;
				$str .="<img src='wp-content/uploads/". $file ."' alt='" . $file . "' />";
				//if ($str) $str .= '|';
				++$i;
			}
		}
	}
	echo $str;
	closedir($handle); // Were not using it anymore
	return $str;
}